1. Field of the Invention
The present invention relates to an improved solid state image pickup device. More specifically, the invention relatives to the solid state image pickup device for correcting dispersion of sensitivity of pixels.
2. Description of the Related Art
In recent years, in order to enlarge a dynamic range in a solid state image pickup device having a photoelectric converting element such as a photodiode, a solid state image pickup device for outputting an electric signal which is converted natural logarithmically with respect to an incident light quantity is suggested. The applicants of the present invention suggest the solid state image pickup device for performing a logarithmically converting operation in Japanese Patent Application Laid-Open No. 11-313257 (1999) and the like. In such a solid state image pickup device, however, dispersion of sensitivity occurs between pixels due to a difference in a threshold voltage of MOS transistors for performing the photoelectric converting operation.
A differential amplifier is, therefore, provided accordingly, so as to output an image signal at the time of an image pickup operation and a noise signal showing the dispersion of sensitivity of each pixel. The differential amplifier subtracts the noise signal from the image signal so as to correct the dispersion of the sensitivity of each pixel.
In the solid state image pickup device for correcting the dispersion in such a manner, at the time of the structure in Japanese Patent Application Laid-Open No. 11-313257 (1999), since a voltage to be applied to a capacitor for integrating an electric signal obtained by the photoelectric conversion is constant, obtained image signal and noise signal are as shown in FIG. 8. FIG. 8 is a timing chart showing a change in a voltage value of a capacitor C at the time of integrating and reading the image signal and the noise signal. The image signal in FIG. 8 is obtained when a quantity of incident light to the pixels is minimum, and the MOS transistor composing the solid state image pickup device is a P-channel MOS transistor.
With reference to FIG. 8, since the image signal is obtained when a quantity of incident light to the pixels is minimum, a voltage value Vcs1−Vcn1, which is a difference between a voltage value Vcs1 at the time of outputting the image signal and a voltage value Vcn1 at the time of outputting the noise signal, becomes a voltage value as an origin of an offset voltage of the image signal from which a noise is removed. In recent years, however, according to refining of a semiconductor chip, a power source voltage to be applied to the solid state image pickup device is lowered, and thus an effective range of a voltage for operating an a output buffer and an AD converter in the solid state image pickup device becomes narrow. Since integrating time of the image signal is nearly 10000 times as long as integrating time of the noise signal, it is found from FIG. 8 that the voltage value of the image signal is lower than the noise signal, and thus the offset voltage becomes high.
In the prior solid state image pickup device, therefore, since a ratio of the offset voltage becomes large in the voltage within the narrow effective operating range, a value Vs−Vn−Vk, which is obtained by subtracting an offset voltage Vk from a differential value Vs−Vn between an image signal Vs and a noise signal Vn becomes low. As a result, a ratio of the voltage utilized for a signal becomes small and its gradation might be deteriorated.